Multifunctionality in mangrove ecosystems: the social network approach

Présentation avec posterinfo:eu-repo/semantics/publishedYoung Marine Scientists’ Day Vlaams Instituut voor de Zee (VLIZ), 24 février, Brugge, Belgiqu

Connectivity of <i>Scylla serrata</i> in Kenya and the Indian Ocean

Few studies were conducted on the genetic population structure and gene flow of marine organisms. Some were carried out in different places in and around the Indian Ocean, including Indonesia and the Red Sea (Froukh and Kochzius 2007, Khalaf and Kochzius 2002, Kochzius and Nuryanto 2008, Fratini and Vannini 2002). All these studies have been conducted on the basis of genetic analyses of mitochondrial markers. Some species like Tridacna crocea (Indonesia) or Larabicus quadrilineatus (Red See) show very strong genetic population structures indicating restricted gene flow and isolation. The same species, present in different isolated areas could evolve into different populations. Thereby, the study of genetic population enables us to identify the population structure and could help to infer the population movement and connectivity between remote sites. For instance, Nuryanto and Kochzius (2008) found 4 different populations of giant clam in the Indian Ocean, west Pacific and Red Sea: (1) Red Sea, (2) Eastern Indian Ocean and Java Sea, (3) Indonesia through-flow and seas in the East of Sulawesi, and (4) Western Pacific. Even though the sample site in the Western Pacific is geographically closer, the haplotypes from there (3) are more divergent from the ones found in the central Indo-Malay Archipelago (1) than the haplotypes from the much more distant Red Sea (4). This could be explained by the low sea level during glaciations, which lead to a strong ecological barrier between the Indian Ocean and the west Pacific, and by the oceanic currents that restrict gene flow in that direction. To better understand the population structure and the gene flow in and around the Indian Ocean, we propose to take more samples in a place where there were none: eastern African coast and more particularly in Kenyan coast. In addition, these samples allow studying the connectivity and the genetic structure of Kenya’s marine organisms and could be related to the structure and to the distance of mangrove forests, on which these species are known to depend for food and refuge. In addition, the samples could be linked with a Kenyan mangrove GIS to infer the connectivity of marine organisms. The starting hypothesis that we will check is that there is some genetic diversity amongst the organisms. In other words, some different haplotypes and therefore different isolated populations exist along the coast. The methodology consists in taking samples by fishing, diving or snorkeling in different places along the Kenyan coast. Collaboration with local fishermen is also considered. The results of this study will help to understand the dispersal behaviour of the Mangrove mud crab Scylla serrata at two different scales: a small scale (Kenya) and a larger scale (in and around the Indian Ocean)

Homing in the mangrove swimming crab <i>Thalamita crenata</i> (Decapoda: Portunidae)

Thalamita crenata is a swimming crab which commonly lives in the shallow waters of sheltered shores of the whole Indo-Pacific region. On the Kenyan coast, this species colonises the intertidal plateau in front of the mangrove. This crab proved to be faithful to a familiar area in which it visited a small number of known holes where it hides during low tide (Vezzosi et al., in prep.). The aim of this study was to test the homing ability of T. crenata after being passively displaced both within the supposed familiar range (internal homing) and outside it (external homing)

Feeding specialization and activity in certain mangrove crabs

Decapoda is surely one of the most important taxa, both regarding the number of species and individuals, in the macrofauna of the mangrove ecosystem (Macnae, 1968; Macintosh, 1988). For this reason, knowledge about their feeding habits and diet is of primary importance for understanding the energy flow typical of this ecosystem.In Mida Creek and Gazi Bay (Kenya) we collected and then analysed the stomach contents of about 350 crabs belonging to the most common mangrove crab species (excluding the Uca species whose micro-algal feeding habits were already known): Cardisoma carnifex, Epixanthus dentatus, Eurycarcinus natalensis, Metopograpsus oceanicus, M. thukuhar, Sesarma elongatum, S. guttatum, S. leptosoma, S. meinerti, S. ortmanni, Thalamita crenata.Species were divided into classes according to the animal/vegetable matter ratio of their stomach contents.The average stomach fullness proved to be correlated with the degree of vegetable content, being lower in the carnivorous species.Stomach analysis showed that only a few species were strictly herbivorous (mostly feeding on leaf litter) or strictly carnivorous (ambush predators of molluscs or crabs). Instead, magrove crabs seemed to be generally omnivorous, probably due to the low energy and protein content of the food commonly found and to the difficulties in catching suitable prey.For Thalamita crenata, a predator inhabiting the intertidal platform in front of the mangroves, it was also possible to analyse the feeding activity in relation to the different conditions of tide and light. Its feeding activity was higher during the day than at night, and it also seems to feed during low tide, probably catching prey during ebb tides and consuming them when it hides in its shelter